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Enhanced Sensitivity of Patterned Graphene Strain Sensors Used for Monitoring Subtle Human Body Motions Sang Woo Lee, Jung Jin Park, Byung Hyun Park, Sung Cik Mun, Yong Tae Park, Tae Seok Seo, Woo Jin Hyun, and O Ok Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01551 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017
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ACS Applied Materials & Interfaces
Enhanced Sensitivity of Patterned Graphene Strain Sensors Used for Monitoring Subtle Human Body Motions Sang Woo Lee, 1,‡ Jung Jin Park,1,‡ Byung Hyun Park, 2 Sung Cik Mun,3 Yong Tae Park,4 Tae Seok Seo,5 Woo Jin Hyun,3,* and O Ok Park1,* 1
Department of Chemical and Biomolecular Engineering (BK21+ Graduate Program) Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
2
Organic analysis PJT, Corporate R&D, LG Chem. Ltd., 188 Munji-ro, Yuseong-gu, Daejeon, 34122, South Korea 3
Department of Chemical Engineering and Materials Science University of Minnesota 421 Washington Avenue S.E., Minneapolis, Minnesota 55455, United States. 4
Department of Mechanical Engineering Myongji University 116 Myongji-ro, Cheoin-gu, Youngin, Gyeonggi-do, 17058, Republic of Korea 5
Department of Chemical Engineering, College of Engineering Kyung Hee University 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do, 17104, Republic of Korea ‡ These authors contributed equally to this work *E-mail:
[email protected] *E-mail:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT
With the growth of the wearable electronics industry, structural modifications of sensing materials have been widely attempted to improve the sensitivity of sensors. Herein, we demonstrate patterned graphene strain sensors, which can monitor small-scale motions by using the simple, scalable and solution-processable method. The electrical properties of the sensors are easily tuned via repetition of the layer-by-layer assembly, leading to increment of thickness of the conducting layers. In contrast to non-patterned sensors, the patterned sensors show enhanced sensitivity and the ability to distinguish subtle motions, such as similar phonations and 81 beats per minute of pulse rate.
KEYWORDS stretchable electronics, strain sensor, graphene pattern, layer-by-layer assembly, human motion monitoring.
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1. Introduction The development of components for wearable devices has been of much interest in the healthcare, virtual reality, and robotics industries.1-8 Of particular interest is the strain sensor, which can convert strain into electrical signals and is an important part of devices that interact with diverse human motions. However, conventional strain sensors are composed of metal or semiconductor materials, which are rigid and brittle and are therefore limited to a strain detection range of only a few percent.9-10 To fabricate wearable, stretchable strain sensors showing sensitivity and stability comparable to those of conventional sensors, the use of various potential conducting materials in stretchable strain sensors, such as metal nanoparticles,1, 3 conducting polymers,11-12 carbon nanotubes (CNTs),7, 11, 13 and graphene,5-6, 8, 14-21
have been explored in previous investigations.
Graphene, a two-dimensional (2D) carbon nanomaterial, shows outstanding electrical and mechanical properties due to its hexagonally packed sp2-hybridized carbon atoms.5, 14-15, 22-23 It also possesses biocompatibility and piezoresistivity, all of which make it a promising material for use in strain sensors.15 2D graphene strain sensors have been shown to be more sensitive at motion detection than other sensors with conducting materials of a high aspect ratio, such as CNTs and metal nanowires.8 Thus, they can be used distinguish subtle respiratory and pulse strains.5-6, 21 To improve the sensitivity of the sensors through structure modification, many researchers have intensively investigated patterned strain sensors.2,
5-6, 12, 21
Introducing a patterned
structure into the sensor allows it to detect minute human motions with improved accuracy2425
because a high pattern density can improve the sensitivity of the sensor.5, 25 The patterned
sensors also possess a good signal-to-noise ratio.25 However, fabricating patterned sensors is difficult, since their patterned structures generally are prepared from a complex and expensive process, such as chemical vapor deposition (CVD) on a patterned substrate2, 5-6, 183 ACS Paragon Plus Environment
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or in situ polymerization on a patterned membrane.12 Furthermore, an additional etching
step that removes redundant metal substrates2,
5-6, 14, 16, 18-19, 21, 26
or polymer layers2,
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increases the difficulty and complexity of the process. In general, patterned graphene sensors exhibit improved sensitivity in comparison with other sensors.5-6, 16, 18-19, 21 This is mainly due to the few 2D mesh-patterned layers of CVDgrown graphene, which exhibit good electrical properties. However, the sensors using CVDgrown graphene could be fabricated through using the specific metal substrates with carbon solubility, such as copper mesh, under high-temperature condition, and hence additional metal etching process using acid is required.5-6, 14, 16-19, 21, 27-28 This method is expensive and complicated, therefore making it unsuitable for practical applications. Moreover, it is hard to increase the thickness of the graphene layer in relation to the sensitivity of the sensor, since the graphene thickness is dependent upon the intrinsic carbon solubility of the substrate materials.29-30 Herein, we demonstrated the fabrication of patterned graphene strain sensors with enhanced sensitivity using a computer numerical control (CNC) milling process, as well as LbL assembled graphene coating on the substrate. A grid-patterned sensing material could be realized through the following procedure: First, square pieces were carved from the polymer film to form a grid-patterned polymer substrate. The grid-patterned graphene film was then obtained by LbL assembly on the substrate. Finally, the coated graphene layer was transferred to stretchable materials such as poly(dimethyl siloxane) (PDMS). The thickness of the sensing materials was easily controlled on a molecular scale by repetition of the LbL cycle,31-32 which could tune the sensitivity of the sensor.23 The patterned polymer film could also be subsequently reused for the fabrication of other sensors, after the graphene was transferred to stretchable materials. More importantly, pattern of the polymer substrate could be easily designed in any shape and size by using computer software, which enables us to 4 ACS Paragon Plus Environment
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control dimension and sensitivity of the sensors for various applications. Therefore, all processes in this method were facile, cost-effective, and scalable. To investigate the advantages of using the grid pattern, we fabricated two types of graphene strain sensors by using either grid-patterned or flat substrates. Depending on the existence of the pattern, the graphene sensors exhibited different performance in sensing subtle human motions.
2. Experimental Section Preparation of the GNP/PSS dispersion and PVA solution Graphene nanoplatelet (GNP) powder (No. N002-PDR; XY
